C++: Initial version of CFG.qll

This implements calculation of the control-flow graph in QL. The new
code is not enabled yet as we'll need more extractor changes first.

The `SyntheticDestructorCalls.qll` file is a temporary solution that can
be removed when the extractor produces this information directly.

cpp/ql/test/library-tests/qlcfg/constructorinitializer.cpp

@@ -0,0 +1,9 @@
+class C {
+	public:
+		C(int x, int y);
+};
+
+void f() {
+	int i, j, k, l;
+	C c(i + j, k - l);
+}